The present invention relates to an organic photoelectric conversion element using a photovoltaic effect of an organic semiconductor material.
An organic photoelectric conversion element supplies electric power outside the element by making use of the photovoltaic effect of an organic semiconductor sandwiched between electrodes. Since such an organic photoelectric conversion element has many advantages such as a lower energy cost for manufacture and a lower environmental load for disposal than those of, for example, photodiodes using conventional inorganic semiconductors, studies for practical application are being made.
There are many types of the organic photoelectric conversion element, including a wet type reported by Gratzel et al. (See, for example, non-patent literature 1.), a type based on a stacked structure (See, for example, non-patent literature 2.), and one based on the mixture of an electron donating organic matter and an electron accepting organic matter (See, for example, non-patent literature 3.).
Here, explanation will be given on the configuration of a conventional organic photoelectric conversion element. FIG. 6 is a cross-sectional view of the essential part of a conventional organic photoelectric conversion element. In FIG. 6, 1 indicates a substrate, 2 an anode, 3 a photoelectric conversion region, 4 an electron donating layer comprising an electron donating organic material, 5 an electron accepting layer comprising an electron accepting material, and 6 a cathode, respectively.
The organic photoelectric conversion element, as depicted in FIG. 6, has the anode 2 comprising the light-transmitting substrate 1 made of, for example, glass, and a transparent conductive film, provided on the support 1, made of, for example, ITO fabricated by, for example, sputtering, resistive heating vapor deposition, photoelectric conversion region 3 fabricated in the form of a film, by providing the electron donating layer 4 and electron accepting layer 5 on anode 2 respectively by, for example, resistive heating vapor deposition, and the cathode 6 made of a metal provided thereon by, for example, resistive heating vapor deposition. When the organic photoelectric conversion element having the aforementioned configuration is subjected to light irradiation, light absorption takes place in the photoelectric conversion region 3 to give rise to excitons. In succession, carriers are separated whereby electrons move towards the cathode 6 through the electron accepting material 5 and holes move towards the anode 2 through the electron donating layer 4. Via such process, an electromotive force generates between the two electrodes, and thus it becomes possible to take out an electric power by connecting an external circuit to the element.
[NON-PATENT LITERATURE 1]
M. K. Nazeeruddin, A. Kay, I. Rodicio, R. Humphry-Baker, E. Mueller, P. Liska, N. Vlachopoulos & M. Graetzel, “Journal of the American Chemical Society”, 115, 1993, p. 6382–6390 .
[NON-PATENT LITERATURE 2]
P. Peumans & S. R. Forrest, “Applied Physics Letters”, 79, 2001, p. 126–128.
[NON-PATENT LITERATURE 3]
G. Yu, J. Gao, J. C. Hummelen, F. Wudl & A. J. Heeger, “Science”, 270, 1995, p. 1789–1791.
To enhance the efficiency of an organic photoelectric conversion element, effective light absorption resulting from matching the light absorption characteristic of the photoelectric conversion region with the spectrum of the incident light, investigation of material design as well as device structure enabling effective charge separation, and further efficient carrier transport caused by the enhancement of the carrier mobility of the constituent material are necessary. And, all of these techniques are being devotedly studied.
Among them, increase of the amount of light absorption by matching the light absorption characteristic of the photoelectric conversion region with the spectrum of the incident light is a very important factor for effective conversion of light energy to electric power. However, there has been a problem that an ordinary organic material is difficult to absorb the light covering a broad wavelength range by itself, and thus cannot generate carriers efficiently, leading to the deterioration of conversion efficiency.